1. Field of the Invention
The present invention relates to a lens barrel that performs focusing or zooming by moving a lens holding member in the direction of the optical axis with this move being interlocked with the rotation of a rotating cylinder.
2. Description of the Related Art
There is, in the known art, an automatic focusing system in which a focal point detection signal obtained by a focal point detection device within the camera body is communicated to a CPU that is provided inside the lens barrel and the CPU electrically drives the focusing optical system so that a specific focus state can be achieved. FIG. 8 shows an example of the prior art of lens barrel used in this type of automatic focusing system.
In FIG. 8, a numeral 1 indicates a fixed lens barrel that is provided with the mounting section 1a for mounting the lens barrel onto the camera body. On the circumferential surface of the small diameter section 1b of the fixed lens barrel, a guide groove 1c is provided, running in the direction of the optical axis, and the lens holding portion 1d for holding the fixed lens group L1 is formed in the rear section of the inner circumferential surface of the fixed lens barrel 1. On the front portion of the inner circumferential surface of the small diameter section 1b, the lens holding cylinder 2 for holding the moving lens group L2 is fitted on in such a manner that it can travel in the direction of the optical axis and the spring 3 is provided between the lens holding cylinder 2 and the lens holding portion 1dof the small diameter section 1b. The spring 3 applies a constant force to the lens holding cylinder 2 in the forward direction along the optical axis of the lens group L2.
A numeral 4 indicates the cam cylinder that is externally fitted on to the tip of the small diameter section 1b in such a manner that it can rotate and on the inner circumferential surface of the cam cylinder 4, a fitting groove 4a and a cam groove 4b are formed in the direction of the circumference. A plurality of pins 1e protrude from the tip of the small diameter section 1b are inserted into the fitting groove 4a and with this, the cam cylinder 4 rotates while its movement in the direction of the optical axis is prevented. The cam follower 2a which protrudes from the lens holding cylinder 2 is inserted into the cam groove 4b of the cam cylinder 4 by passing through the guide groove 1c of the small diameter section 1b.
A segment gear 4c is formed at the rear end on the external circumferential surface of the cam cylinder 4 and the pinion gear 5 which fits with the segment gear 4c is linked to the focusing motor 6 via a gear train (not shown). A numeral 7 indicates an encoder for lens travel distance detection which consists of the pattern section 4d formed as a part of the external circumferential surface of the cam cylinder 4 at the forward end of the cam cylinder 4 and the detection section 8 that detects the patterns of the pattern section 4d. With this encoder 7, the amount of rotation of the cam cylinder 4, that is, the travel distance of the lens is detected and the results of the detection are input to the CPU (not shown) within the lens barrel.
The lens barrel that is structured as described above is mounted on the camera body (not shown) via the mounting section 1a. When the release button of the camera body is pressed halfway down, the focal point detection signal which has been calculated in the focal point detection device of the camera body is input to the CPU within the lens barrel. The CPU supplies a drive signal to the focusing motor 6 in response to the input focal point detection signal and with this, the focusing operation is performed. The rotation of the focusing motor 6 is communicated to the cam cylinder 4 via a reduction gear train (not shown), the pinion gear 5 and the segment gear 4c to cause the cam cylinder 4 to rotate around the optical axis. With the rotation of the cam cylinder 4, the cam groove 4b moves and the cam follower 2a is driven along the guide groove 1c to cause the lens holding cylinder 2, that is, the moving lens group L2 to move, relative to the fixed lens group L1, in the direction of the optical axis. When the lens travel distance as detected by the encoder 7 reaches the calculated lens travel distance, the CPU stops the focusing motor 6.
The spring 3 which is provided between the lens holding cylinder 2 and the lens holding portion 1d eliminates the play between the cam cylinder 4 and the lens holding cylinder 2 in the direction of the optical axis. As described above, with the type of lens barrel in which the rotation of the cam cylinder 4 is detected by the encoder 7 to determine the lens travel distance, if there is any play between the cam cylinder 4 and the lens holding cylinder 2 in the direction of the optical axis, an discrepancy is generated between the actual lens travel distance and the output of the detection from the encoder 7 to lower the focusing accuracy. To deal with this problem, force is applied by the spring 3 to the lens holding cylinder 2 in the direction of the optical axis to press the cam follower 2a against the wall surface of the cam groove 4b. Consequently, the play between the cam cylinder 4 and the lens holding cylinder 2 is removed, improving the focusing accuracy.
In general, in order to improve the focusing accuracy in an automatic focusing system, it is necessary to move the focusing optical system to the correct position and then to stop it with great precision. To achieve this, it is desirable to minimize the elapsed time and distance of the lens holding cylinder 2 traveled from the time when the rotating focusing motor 6 is braked until the time when the lens holding cylinder 2 stops, and to make these constant. The lens stopping characteristics are determined by the moment of inertia of the mechanism, which consists of the focusing motor, the reduction gear train, the cam cylinder, the lens holding cylinder, the lens groups and so forth, as well as the friction torques among the various components of the above mechanism and the like. Therefore, in order to maintain stable and constant lens stopping characteristics, it is necessary to maintain the friction torques among the various components constant and stable.
However, in the structure of the lens barrel in the prior art, since the spring 3 is provided between the fixed lens barrel 1 and the lens holding cylinder 2, the amount of deflection of the spring 3, and, therefore, the force it applies, changes greatly depending upon the position of the lens holding cylinder 2. The force applied by the spring 3 works on the cam cylinder 4 via the cam follower 2a of the lens holding cylinder 2 and at the same time it works on the fixed lens barrel 1 via the cam cylinder 4. Therefore, if the force applied by the spring 3 changes, the friction torques generated on the various surfaces of the cam cylinder 4 and the fixed lens barrel 1 changes. As a result, the lens stopping characteristics change depending upon the position of the lens holding cylinder 2 and this lowers the focusing accuracy.